Asynchronous Transfer Mode (ATM) is an emerging network technology that is designed to transport information between communicating stations in a point-to-point fashion. The interest in ATM is its promise of high bandwidths and quality of service. ATM is a connection oriented architecture, in contrast to network architectures that are structured to broadcast data from the source to the destination. In ATM, the source negotiates a connected path to the destination before it proceeds to transmit its information to the recipient. ATM protocols (or "rules," usually implemented in software) define the communications necessary to establish the connection. An ATM attached device has an ATM address in addition to any other network addresses it might have, depending on the particular ATM configuration within which it is incorporated. Some possible configurations will be described subsequently. Once a connection is established, the source station transmits its traffic only to the destination.
In contrast to connection oriented architectures are broadcast networks. In these, data is sent from a source station to a destination station by broadcasting it to all addresses where the recipient plucks it off the network while the other stations on the network ignore traffic not bound for them. Broadcast architectures provide one motivation for structuring a "network" as a set of interconnected subnetworks or "subnets."
In a large network, the proliferation of broadcast packets would overwhelm the network. Although a particular network may start out as a freestanding Local Area Network (LAN), eventually end-station users will probably want to avail themselves of the services available on other networks, and look to connect "their network" with other "networks." When this occurs, it is intuitive, as well as more precise, to view the resulting network structure as a set of subnets within a larger network, for example, an "intemetwork." However, a station on one internetworking subnet that wishes to communicate with a destination on another subnet can only do so if there is connectivity between the subnet in which the source resides and the subnet in which the destination resides.
Communications methodologies between subnets are usually termed "layer-3" protocols. This refers to the layered architecture networking model of the International Standards Organization (ISO). This model is illustrated in FIG. 1. Layer-3 may sometimes be referred to as the "network" layer, and is equivalent to the "internetworking" layer in the TCP/IP model.
Connectivity between layer-3 subnets is provided by a device termed a "router." When a source station on one layer-3 subnet wishes to communicate with a destination station on another layer-3 subnet, it broadcasts the data in the usual way. However, now it is the router that plucks the data packets off the first subnet and forwards it to the destination station via the destination station's layer-3 subnet to which the router is also attached.
Numerous types of networks coexist in the data communications industry. In addition to ATM, there may be LANs, Wide Area Networks (WANs), and others. There is a need in the industry for interconnection between different network architectures and, in particular, users of preexisting LANs have a need to connect to emerging high speed network technologies, such as ATM. The need for incorporating or interfacing preexisting networks (more precisely subnetworks) into an ATM environment has led to the specification of several methodologies to support preexisting network architectures within ATM.
One such methodology is the emulated LAN (ELAN) which simulates classical LAN protocols in an ATM environment. (Classical LAN protocols, for example Ethernet and Token Rings, are referred to as legacy LANs.) The protocols that provide the specification for ELANs are called LAN emulation (LANE). Layer-3 protocols run on top of ELANs just as they do in legacy LANs. Hosts attached to the ELAN include emulation software that allows them to simulate legacy LAN end stations. Such hosts are called LAN Emulation Clients (LEC). The LEC software hides the ATM from the LAN protocols within the LEC device, and those protocols can utilize a LEC as if it were a legacy LAN. A LEC can also provide a standard LAN service interface in a LAN Switch that is usable to interface a legacy LAN with an ELAN.
Communication between LECs on an ELAN can be effected over the ATM. Each LEC has a physical, or Media Access Control (MAC) address associated with it, as well as an ATM address. For one LEC on a ELAN to communicate with another, it must obtain the ATM address of the destination LEC, given the destination MAC address. This address resolution is mediated through a LAN Emulation Server (LES). The source LEC issues a LANE Address Resolution Protocol Request (LE_ARP_Request) to the LES. Provided the destination station has previously registered its MAC address, ATM address pair with the LES serving the ELAN, the LES returns the ATM address of the destination to the requesting LEC in an ELAN Address Resolution Protocol Reply (LE_ARP_Reply). The source LEC can then use the ATM address to establish a connection to unicast data to the destination, a so-called data-direct Virtual Channel Connection (VCC), and transmit its data to the destination thereon.
LANEs are also specified for emulation of source routed LANs, for example Token Rings, as well as nonsource routed LANs, such as Ethernets. In source routed LANs, packets being sent to a destination station contain a Routing Information Field (RIF) that includes a path from source to destination that is an ordered set of route descriptors, ring and bridge numbers, forming the route between source and destination station. Operations performed on MAC address described hereinabove are correspondingly performed on the RIF in a source routed ELAN.
In the source-route bridged network, a source routed frame contains a RIF. The RIF includes an ordered list of ring and bridge numbers through which the frames are to pass from the source station to the destination station. Typically, the source station determines the route to the destination station by broadcasting an explorer frame. Bridges add the routing information to the RIF before forwarding the explorer frame. When the explorer frame reaches the destination, the destination station sends a response to the source station. The response contains the complete RIF that the source station then includes in subsequent frames addressed to that destination. Bridges make frame forwarding decisions based on the RIF.
Source routed LAN stations are connected to edge devices, for example LAN bridges, that bridge traffic between the legacy LAN ports and ELAN ports on the switched ATM network. However, traffic is still routed via the source routed path specified in the RIF, because the bridge does not have the information it needs in order to establish direct layer-2 ATM connections.
In order for a network employing source route bridging to take advantage of the speed and efficiency associated with route switching, there is a need in the art for a mechanism to enable source-route bridged networks to participate in route switched networks.